396 l78 - Team Camaro Tech
Engine General Engine Discussion.

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post #1 of 7 (permalink) Old Feb 26th, 17, 06:00 PM Thread Starter
Rod
 
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Location: Manitoba (MB)
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396 l78

I have a 396 Hi-perf engine its a 2 bolt main yet i thought the Hi perf engines were 4bolt mains 3969854 engine code
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post #2 of 7 (permalink) Old Feb 26th, 17, 06:44 PM Thread Starter
Rod
 
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This is the heads casting number 3933148 any idea what i have . It is stamped behind the timing cover Hi perf 6 pass engine build date E12/70
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post #3 of 7 (permalink) Old Feb 26th, 17, 07:07 PM
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Dave
 
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Re: 396 l78

ALL High Performance big blocks are four bolt main. The block you have is a PASS for passenger car block two bolt main. The number of main bolts isn't as much of an issue with a 396 due to it's small restrictive bore size. You have the equivalent of the 305 block of small blocks.

Here is a 305 block with a 2.02 intake valve 350 head bolted to it.



As many will yell you the 305 is a great engine used in millions of Chevy vehicles. The same can be said about the 396. I used to throw those motors in the scrap metal pile then, as I do with a 305 engine now. The bore is too small to be useful. Chevy engineers had to notch the block to prevent the intake valve from hitting the edge of the cylinder when the valve opened. Because of the small bore the cylinder walls shroud the valves preventing free air flow.



High performance is all about getting air into and out of an engine. If you are not interested in performance then a 396 is a perfectly adequate motor, but I will add that SBC 400 will drive rings around it for less money, as it is more efficient and weighs nearly 230 pounds less than an all cast iron big block.

By the way the 375 horse 396 had 11.0:1 TRW pistons sticking up out of the block to make that extra horsepower that you gain above the 10.25: compression ratio of the 350 horse motor. Increasing static compression is free horsepower when running the Otto cycle which is why everyone followed Oldsmobile engineers who started the race to high compression engines when the raised the Olds Rocket engine from 7.8 to 8.5:1 back in 1963. The static compression steadily increased year after year through 1970.



Pump gas today will not run on a dynamic compression ratio above 8.5:1 (9.2 static with cast iron heads and 10.2:1 with open chamber aluminum heads). You need race gas to run that high now a days.

Big Dave
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post #4 of 7 (permalink) Old Feb 26th, 17, 09:16 PM
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Devin
 
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Re: 396 l78

Big Dave,
Thanks for the write up. I used to run octane booster (toulene?) and it really helped my 454 open chamber, oval port with TRW L2349F, 2bolt from a 74 Laguna. From memory it yielded 119cc and was 10.25:1. Did I get that math correct? Is adding octane booster the same as using higher octane/race gas?
Thanks in advance

69 Coupe, 454, forged bottom end, approx 10:1 comp ratio, big bumpy cam, Offy 360 tunnel ram, Holley 850, Muncie wide ratio, Hays clutch/flywheel/blowshield, 12 bolt posi, 4.88:1 ratio, radio: none
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post #5 of 7 (permalink) Old Feb 26th, 17, 10:19 PM
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Devin
 
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Re: 396 l78

Correction: my heads are closed chamber.

69 Coupe, 454, forged bottom end, approx 10:1 comp ratio, big bumpy cam, Offy 360 tunnel ram, Holley 850, Muncie wide ratio, Hays clutch/flywheel/blowshield, 12 bolt posi, 4.88:1 ratio, radio: none
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post #6 of 7 (permalink) Old Feb 27th, 17, 06:50 AM
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Dave
 
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Re: 396 l78

Quote:
Originally Posted by rod66 View Post
I have a 396 Hi-perf engine its a 2 bolt main yet i thought the Hi perf engines were 4bolt mains 3969854 engine code
Quote:
Originally Posted by rod66 View Post
This is the heads casting number 3933148 any idea what i have . It is stamped behind the timing cover Hi perf 6 pass engine build date E12/70
The block you have is the same block used for the 350hp 396. 2 bolt. I'm sure others as well. You can make plenty of power with it to have fun, but it just isn't as easy/cheap as with other big blocks. My born with 350hp/396ci is also an 854 block, but 290 heads.
I have almost the same set up as yours for my spare motor that I was running all last summer. 854 block +.030 (.060 over 396) with 148 heads. It was plenty of fun. The only reason I pulled it was a lifter tick. That spiraled into a 494ci...
If you really want to know what your motor came from, look on the deck pad under the alternator. You'll see a date and a code like "TLM" which is from a truck. With 148 heads, a truck motor is probably what you have.

'69 SS396 X66 L34 08D M21 BS
460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

Semper Fi!

"The galleries are full of critics. They play no ball, they fight no fights. They make no mistakes because they attempt nothing."
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post #7 of 7 (permalink) Old Feb 27th, 17, 06:52 AM
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Dave
 
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Re: 396 l78

Adding toluene is the same as high octane gas because that is what the refiners used to raise the octane. Gas isn't a single compound but a blend of many chemicals and that smell we associate with gas is due to the lighter aromatics evaporating out of the mixture. Toluene (lacquer thinner) and xylene are two of the most common chemicals added to gasoline to raise 87 octane stock to near 100 octane. To get above 106 octane you need a witches brew of exotic chemicals; which is why it has a best by date of last week and cost so much. As soon as the container is opened they start to evaporate away as many have boiling points that are barely above room temperature up north: so that in parts of Texas and south Florida the container is bulging with the vapor pressure that rushes out when you open it.

Stock open chamber heads where cast at 119cc to 121cc. They were opened up to form a semi-hemispherical combustion chamber (filled with half of a hemispherical piston dome back in 1970-'71) to unshroud the valves for better breathing. At 2.30" inches as big and as heavy as they are they are too small to support the actual displacement of current big blocks. Compare the area of a 2.02" headed SBC 302 in terms of area divided by displacement, to a 2.30" 632 to BBC (anything bigger and you are no longer using stock heads) and you will see that the "Nail Head" Buick from back in the early sixties could claim to be the big valved engine.

Having huge rectangular ports has been proven to not work as well at allowing a BBC to breath as having smaller oval port heads with bigger valves installed in them. Back in the early seventies racers learned that taking the 2.19" valves out of their rectangular port HiPerf heads and installing them in their oval port PASS heads made their 427 engines run faster (made more horsepower). This is why HiPerf heads are so cheap today, no one other than someone with a restoration project has them installed on an engine under 500 cubes. It takes a 540 or bigger BBC to actually make use of a port with that volume (even though rectangular shape is no longer in favor; with 320 cc and larger port intakes today are all oval shaped).

A 375 horse 396 built with 12.5:1 pistons will make 425 horsepower for circling the high banked oval tracks of the Southeast. But on the street the HiPerf heads on a 396 results in an engine that runs like a dog. The 325 horse 396 will beat up on it on the street. The 375 horse won't begin to make power until you rev it above 4,600 RPM.

Big Dave
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